1. Field of the Invention
The present invention relates to apparatus for reproducing a PCM signal recorded on magnetic recording medium through helical scan of a slant azimuth method, and more particularly to a clock signal regenerator suitable for high speed data search operation of a rotary magnetic head type digital audio tape recorder or the like by which audio signals are reproduced in the form of PCM signals.
2. Related Art
In a digital audio recording-reproducing apparatus including such as a rotary magnetic type digital audio tape recorder (R-DAT), information is recorded on a recording medium such as a magnetic tape in the form of digital signals in synchronism with clock signals. In general, clock signals are not recorded on the recording medium so that it is necessary to regenerate clock signals based on the signals reproduced from the recording medium. The clock signals are regenerated generally by using a phase-locked loop (PLL) circuit by which transitions of the reproduced signals are subjected to phase comparison with a voltage-controlled oscillator VCO output.
In the reproduction of the recorded PCM digital signal on a two head-type apparatus, the signal is alternately read out from a magnetic tape via each one of the two magnetic heads mounted to a rotary drum. Such alternative switching is made by a head-switching signal generated from the rotary drum. The read-out PCM digital signal burst generated from each of the two heads is applied to a PLL circuit to regenerate a clock signal therefrom. The PLL is locked in frequency to each of alternately generated PCM signal bursts from the two heads. For the purpose of improving the recording density, a known slant azimuth recording manner is taken. That is, a different slant azimuth is assigned to each of the two magnetic heads to perform the guard- band-less recording. In the magnetic heads with different azimuth angles, the relative speed change degree occurring when the running speed of a magnetic tape is changed for a high speed search operation is slightly different for each magnetic head. The PLL circuit will alternately track in frequency to each of the frequency-different output signal bursts from the two heads. Accordingly, since at the beginning point of the output burst from one head there is a frequency error or shift from the locking state to the other head, it takes some time to get a pull-in state in the PLL circuit for the output burst from said one head. Due to this time, it becomes impossible to regenerate a proper clock signal and thus to discriminate the read-out PCM digital signal for PCM signal encoding at the beginning of the output signal bursts.
Other problems in the conventional clock regenerators will become clear in relation to the description below of the embodiment of the present invention.